If that came off, two things would have happened - the huge influx of unmetered air would have caused your engine to run at full revs, and your braking ability reduced by at least 80%. So thank god you caught it.
No it wouldn't. The engine would stall at the first (and every) opportunity.
Agree about the brakes, they would be very wooden and ineffective, although the brake servo would hold vacuum for a while if it fell off mid drive.
I don't think you're wrong, but I do think it depends on how loose the pipe was. If it didn't totally detach, but let enough air in, the revs could rise.
But it's academic.... main thing it isn't been caught early, and everyone is ok
You just have to consider where is the extra fuel going to come from. The ECU doesn't know about the extra air, so won't inject extra fuel to burn the extra (leaking) air. This will result in a lean mixture. Fuel/Air won't burn if the mix is greater than about 20:1 - normal stoichimetric is about 14.7:1. So if the air leak makes up more than about 30% of the air going into the engine, there won't be enough fuel to sustain combustion, and the engine will stall. This is most likely to happen when the throttle is closed - so at idle, or during overrun/deceleration. To keep the engine running you are likely to have to apply some throttle whilst sitting at traffic lights.
At larger throttle openings the engine will run, but it'll run leaner than desired. The volume of air passing through the throttle bodies (and more importantly over the MAF sensor) will be (much) larger than that through the air leak, so the ECU will inject enough fuel to sustain combustion. It'll still be running leaner than ideal though coz there won't be enough fuel to burn all the air, and at high power settings this could be damaging to the pistons. Lean mixtures burn hotter.
Lambda sensors don't despond quickly enough to catch this sort of behaviour - they're basically used to adjust medium and long term fuel trims. They won't learn quickly enough to compensate for air leaks, and even if they did, they only have enough authority to change the injector duration by about +/- 15%.
So on MAF (Mass Air Flow) based systems, air leaks tend to cause lean running, and stalling at low throttle openings. On a MAP (Manifold Absolute Pressure) sensor setup things are different - and the engine would/could run away, but Omega B and most Vauxhalls are MAF setups.